The sub-plot about Marissa getting a tattoo was included after Mischa Barton actually did get a tattoo. Because she didn't get their permission first, the producers were initially very angry, but eventually decided to write it in and create the story about Marissa and Alex visiting the tattoo parlour.
In the show's pattern of breaking the fourth wall/inside jokes, Seth mentions the film 'Summer Lovers' - his fictional father played by Peter Gallagher starred in the film.
Lindsay has a "Dennis Kucinich for President" bumper sticker on the back of her car